How are the values for the `sex` feature in sklearn Diabetes dataset obtained?

Question

I'm just starting out with using sklearn for my own Machine Learning project and I'm using sklearn's built-in "Diabetes" dataset.

While performing data exploration on the features, I noticed something a bit confusing to me about the sex feature. Here's the hist plot:

Now there are 2 things I do understand here:

1. The binary histogram makes sense, there are in this dataset 2 distinct 'sexes' of male and female.
2. Them being numerical also makes sense, as it appears all features in this dataset have already been 'normalized'.

What I don't understand is why the values are the way they are? (See below for what the values are)

>>> from sklearn import datasets
>>> diab_df = datasets.load_diabetes(as_frame=True)
>>> features = diab_df['data']
>>> features.sex.unique()

array([ 0.05068012, -0.04464164])

How are these numbers derived? At first, I thought it could be some sort of stratified sampling, where if the true population distribution is say, 53% male, 47% female, then I'd maybe expect to see the values in this hist to be -0.47 & 0.53 or something?

Answer 1

The data description says:

Note: Each of these 10 feature variables have been mean centered and scaled by the standard deviation times n_samples (i.e. the sum of squares of each column totals 1). https://www4.stat.ncsu.edu/~boos/var.select/diabetes.html

For more information see: Bradley Efron, Trevor Hastie, Iain Johnstone and Robert Tibshirani (2004) "Least Angle Regression," Annals of Statistics (with discussion), 407-499. http://web.stanford.edu/~hastie/Papers/LARS/LeastAngle_2002.pdf

from sklearn import datasets
print(datasets.load_diabetes())